不定高居中布局flex.html

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .parent {
            width: 50px;
            height: 200px;
            background-color: #ccc;
            display: flex;
            align-items: center;
        }

        .child {
            background-color: #222222;
            color: #fff;
            width: 100%;
        }
    </style>
</head>
<body>
<div class="parent">
    <div class="child">DEMO</div>
</div>
</body>
</html>